Mayoral hopeful John Liu won’t know if he’s getting millions of dollars in campaign funds from the city until just weeks before the September primary — a headache that’s sure to hamper his campaign, experts told The Post.

The city Campaign Finance Board isn’t expected to rule on Liu’s request for $3.5 million in matching taxpayer funds until early August, according to a source.

“Right now, he’s behind the eight ball,” the source said.

Since the board’s OK is far from certain — Liu’s fund-raising has been the target of a federal investigation for several years, and his former campaign treasurer was recently convicted of attempted fraud — his campaign can’t be sure when the bucks will stop.

“It’s impossible to manage a campaign like John’s when he has yet to get the feds to say he’s no longer under investigation, and has yet to hear from the CFB if he’s going to get his matching funds,” said Scott Levenson, president of the Advance Group consulting firm.

“Those are two handcuffs that make it next to impossible for any kinds of moves.”

Liu, the city comptroller, has about $2 million left in his campaign account.

But the five-week period between the board ruling and the Sept. 10 primary is the pivotal time for pricey, high-impact television ads — leaving him with hard choices about when and where to sink his cash.

“It’s consequential. He might hold money back from early things — like direct mail and other kinds of organizational things — because he’s holding the money for television,” said Joseph Mercurio, a Democratic media consultant.

“[But] the bulk of the end-game money is television, and if he doesn’t get [the funds] he’s going to have a very small media buy,” he added. “Even if he got the money and he got it late, he could be at a disadvantage in terms of audience size because of the [higher] costs.”

Liu’s campaign did not respond to an e-mail and phone call seeking comment.

But he has previously said that he would resort to fund-raising nationally if he is blocked from receiving the public funds.

Although he is now viewed as politically wounded and well behind in the race, Liu could still have a significant impact.

With five major Democratic candidates tightly bunched, none is expected to hit the 40 percent mark needed to avoid a runoff between the two top finishers.

If Liu falters in the stretch, another candidate — probably Public Advocate Bill de Blasio — could pick up enough of his votes to challenge for the No. 2 spot. De Blasio is now running third or fourth in the polls.

Liu, who hopes to become the city’s first Asian-American mayor, has been getting a fairly good reception at forums across the city thus far.